“Transparent element” means in a non-limiting manner ocular lenses, watch glasses, touch screens with transparent electrodes, for example structured on watch glasses, or even the transparent plates of a liquid crystal display cell or a photovoltaic cell.
An anti-reflective effect is obtained in a known manner by forming, on a transparent substrate (S), a stack including at least one layer of a transparent dielectric material with a high refractive index (H) and a layer of a second dielectric material with a low refractive index (L).
A method of the preceding type is disclosed, for example, in FR Patent No. 2 751 638. A glass substrate is coated via LPCVD from gaseous precursors of a thin film of TiO2 (n=2.35), then with a thicker film of SiO2 (n=1.46), the operation being able to be repeated a second time, which leads to a sequence that can be summarised as “SHLHL”.
A sequence of the same type is also found in U.S. Pat. No. 5,170,291, but employing different dielectrics, one of the examples given including in succession TiO2 (n=2.4)/Al2O3 (n=1.6)/TiO2 (n=2.4)/SiO2 (n=1.48), which corresponds to the sequence “SHL1HL2”.
When the transparent substrate has to include electrodes, for example to form a capacitive touch screen in the glass of a wristwatch, an ITO film, or another transparent conductive oxide (TCO) is deposited on the last dielectric layer, structured with a final coating of a dielectric material with a low refractive index. This method can thus be represented by the sequence “SHL1 (TCO)L2”. It gives very good results, i.e. it enables the electrodes to be practically invisible, but it has several drawbacks. First of all, it is expensive because of the number of different films that have to be deposited by CVD or equivalent methods. It is also difficult to implement, in particular during the electrode structuring step where there is a high risk of the last dielectric film being damaged and thus new problems of interference being created, making certain of the electrode contours still visible. It will be observed finally that the last dielectric layer with a low refractive index, generally MgF2 (n=1.37), has a low abrasion resistance, such that there is also a risk of said layer being damaged during manipulations for mounting the touch screen in the case of a wristwatch.